1. Field of the Invention
The present invention relates to the treatment of rare earth ores, and, more especially, to the processing of rare earth ores with concentrated aqueous solutions of alkali metal hydroxides to recover rare earth values in hydroxide form therefrom.
2. Description of the Prior Art
Industrial processes known to this art for treating rare earth ores generally consist of grinding an ore such as monazite very thoroughly, either dry or in water, then treating the ground ore with a concentrated aqueous sodium hydroxide solution at high temperature. U.S. Pat. No. 2,811,411, for example, describes a process wherein the grinding is carried out in such fashion that in the ground ore 100% of the particles pass through a 74 micrometer mesh sieve and 95 to 98% through a 44 micrometer mesh sieve. The ground ore is then treated with a 30 to 70% by weight sodium hydroxide solution at temperatures of 135.degree. to 220.degree. for periods of 1 to 3 hours, with a weight ratio of sodium hydroxide to ore of 2 to 3. This results in a quite heavy consumption of sodium hydroxide.
Furthermore, the reaction is typically carried out in an autoclave (see, e.g., British Patent No. 2,084,556). The concentration of sodium hydroxide in the reaction medium then decreases upon passage of time, because the sodium hydroxide is consumed in the reaction. The concentration is, therefore, lower upon completion of the reaction than it was at the beginning. The reaction conditions for the ore consequently become more gentle, which does not favor obtaining high reaction yields in a limited time.
To avoid these disadvantages, the known processes use a large excess of reagent (initial weight ratio (NaOH/ore) of 2 or more) and also use a very high initial concentration of sodium hydroxide. The latter requirement involves using anhydrous sodium hydroxide, which is expensive.